Many alloys with high weight percentages of a reactive metal, such as titanium, react with air and most common crucible refractories to the degree that the alloy is contaminated to an unacceptable extent. As a result, it is common to melt such alloys in water cooled, metal (e.g. copper) crucibles using electric arc or induction to generate heat in the alloy charge.
U.S. Pat. No. 4 738 713 is representative of one such melting technique. The patented melting method is very inefficient in the use of electrical power. Moreover, experience with such a method indicates that the amount of melt superheat achievable is limited and sensitive to crucible life. However, the method is in use since the method can use lower cost melt stock than consumable arc melting techniques which require specially prepared melting electrodes of the alloy desired.
Arc melting techniques using water cooled copper crucibles (e.g. see U.S. Pat. No. 2 564 337) can provide higher superheats in melting the reactive alloys. However, arc melting techniques, as well as induction melting techniques, are dangerous due to the potential for explosion in the event of crucible failure wherein cooling water comes into contact with the molten reactive alloy to form hydrogen gas. Both arc melting and induction melting techniques are practiced in remote manner, such as from behind explosion proof walls in specially constructed buildings with blow-out walls. As a result, operation of such cold-wall metal crucibles or furnaces has been costly with good process control difficult to achieve.
Some prior art workers have melted and cast reactive alloys, such as titanium alloys, using calcium oxide crucibles. However, contamination of the alloy melt with oxygen is rapid and, with some alloys containing aluminum, extensive aluminum oxide vapor is evolved in such amounts as to preclude practical operation of traditional casting units by contaminating vacuum systems and chambers associated with the casting unit.
Other prior art workers, see U.S. Pat. No. 3 484 840, have rapidly melted titanium alloys in graphite lined crucibles in order to avoid harmful contamination of the melt. The patented process does not permit accurate control of the melt temperature and excessive melt contamination can occur if the heating cycle is too long. In addition, control of the melt flow out of the bottom of the crucible is difficult since melting of the center portion of a metal disc at the crucible bottom is employed to this end. With this arrangement, the melt flow orifice will vary with the melting rate, charge diameter, and disc size, making control of melt flow difficult.
Intermetallic alloys, such as especially TiAl, have received considerable attention in recent years for use in the aerospace and automobile industries in service applications where their high strength at elevated temperature and relatively light weight are highly desirable. However, these intermetallic alloys contain a majority of titanium (e.g. so-called gamma TiAl includes 66 weight % Ti with the balance essentially Al) which makes melting and casting without contamination difficult and very costly. In order to be adapted for use in such components as automobile exhaust valves, the intermetallic alloys must be melted and cast without harmful contamination in a high production, low cost manner.
It is an object of the present invention to provide a method and apparatus useful for, although not limited to, making intermetallic castings without harmful contamination in a high production, low cost manner especially suited to the requirements of the automobile, aerospace and other industries.
It is another object of the present invention to provide a method and apparatus for making intermetallic castings using a refractory melting vessel and a combination of molten and solid melting stock in a manner to avoid harmful contamination of the melt by reaction with the vessel.
It is another object of the invention to provide a method and apparatus for making intermetallic castings in a low cost manner by virtue of using relatively low cost melting stock which requires reduced energy requirement in order to yield a melt ready for casting into a mold.